Your nephew's daughter's son is your great-great nephew.
Your nephew's daughter is your great niece.
Second cousins once removed.
3rd cousin
They would be great nieces or great nephews.
You would be his Great Uncle and he would be your Great Nephew.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
your second nephew
Nieces are the daughters of your brothers and sisters. Nephews are the sons of your brothers and sisters.
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Nephews and nieces
2nd uncle I believe
You are their great aunt/uncle.